Abstract
Zubair oil reservoir has a significant potential to contribute to the petroleum supply in Iraq. Drilling highly-deviated wells is a commonly used approach to enhance the production in mature fields. Drilling these wells in the Zubair Formation has been a challenge due to severe wellbore instability issues. This study presents how integrated reservoir geomechanics was used for efficient well planning and safe drilling of nearly horizontal wells (75° Inclination) through Zubair depleted sandstone oil reservoir.
In this study, an integrated workflow was implemented mainly to build a geomechanical model using offset well data. The in-situ principal stresses and their orientations were obtained from wireline logging measurements and mini-frac tests. The repeat formation test (RFT) was used to constrain pore pressure. Rock mechanical properties were calculated using empirical correlations that have been derived from laboratory tests on core samples. Mogi-Coulomb failure criteria was used to address safe mud weight for drilling highly-deviated wells successfully. The model was verified with wellbore failure and also with wellbore instability events while drilling operations.
The results obtained from the geomechanical model shows that Zubair sections of planned wells would have narrow mud weight windows. The analyses also emphasized that the shale interbeds in the Zubair section require a mud weight of 1.5g/cc to limit wellbore breakouts. The presented study highlights that the geomechanical model can be applied as cost-effective tools to assess and address existing wellbore instability problems and to guide future well plans for better drilling efficiency with a reduced non-productive time (NPT) by using proper mud weight with respect to the inclination angle and the azimuthal direction.
